Dec. 16, 2011

Science Diction: The Origin Of The Petri Dish

In 1887, Julius Petri invented a simple pair of nesting glass dishes, ideal for keeping specimens of growing bacteria sterile--the 'Petri dish.' Science historian Howard Markel recounts the history of this ubiquitous lab supply, and the serendipitous discovery of the stuff in it, agar.

Shakespeare observed: "Some are born great, some achieve greatness, and some have greatness thrust upon them." The last clause of this famous axiom perfectly describes the singular scientific accomplishment of Julius Richard Petri (1852-1921).

In 1877, while still a young German military physician, Petri was assigned to work for Dr. Robert Koch at the Imperial Health Office in Berlin. Koch, of course, was the Kaiser of bacteriology and was world famous for his work in helping to establish the germ theory of disease as accepted scientific fact. Petri's two-year post under Koch was the equivalent of a bush leaguer inexplicably being called up to play with the New York Yankees.

But in order to pronounce his wondrous findings on a myriad of infectious illnesses, Koch needed to grow bacteria, lots of them. And so he and his assistants focused on creating the most reliable, pure culture laboratory techniques.

The biggest challenge they faced was contamination of their cultures by airborne bacteria. One early approach involved growing bacterial colonies on a flat plate containing a gelatin nutrient medium. The plate was then stored within a heavy glass bell jar to prevent microbes floating about from settling down onto the plate and multiplying. But this method was way too clumsy, especially when trying to observe the colonies through a microscope.

Enter Julius Petri. Influenced by his post at the Koch discovery factory, he tinkered with glassware and in 1887 emerged with a landmark paper, "A minor modification of the plating technique of Koch." That title is a bit of an understatement. Petri’s 300-word article described the now familiar round glass plate with straight sides that contained culture media along with a similarly shaped cover of a slightly wider diameter.

It was simple, practical and brilliant. Not only did Petri’s plate prevent the entry of airborne germs, it was also flat enough to be easily viewed under the microscope for quantitative and morphologic analysis.

Incidentally, it was Fanny Hesse, the wife of another Koch assistant, who suggested a far better culture substrate than gelatin: agar-agar, the Malay term for a polysaccharide derived from red algae seaweed or Rhodophyta. Mrs. Hesse used the stuff when making her summer fruit jellies. In 1881, she convinced her husband, Walter, that agar would not melt under the incubating heat most pathogenic bacteria need to thrive. This seaweed jelly became the perfect mate for the Petri dish, even though we rarely recall Fanny’s name today.

Dr. Petri went on to run a tuberculosis sanatorium. He made his clinical rounds wearing the full dress uniform of a Prussian Army doctor. His uniform included a scarlet red sash wrapped around his morbidly obese waist. Some derided him as a control freak who looked like a three-dimensional map of the equator circling the globe.

No matter. The good doctor’s assignment to Koch's laboratory may have been a propitious stroke of luck. But his mark on history is undeniable. Thrust into the crucible of discovery, he emerged with the dish universally regarded as a "Petri."